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I have a home ventilation switch that I would like to wire a timer to. This switch is located in my upstairs hallway, and when flipped it turns on the furnace fan blower and the main bathroom exhaust fan. However, when I look in the junction box, it's unlike anything I've ever seen.

The switch has four terminals (plus ground), with a standard high voltage wire connected to one side, and a pair of low voltage wires connected to the other side. While I can't trace it through the wall, there is a similar red-sleeved low-voltage wire connected to the control panel on my furnace blower, which I assume is the other end of this wire. However, I have a number of questions:

1) How can there be both high and low voltage wires connected to the same switch? Is there such thing as a switch with a built-in transformer?

2) How/why would the white/neutral wire be connected to the switch? Is this how you terminate a neutral wire when the load device doesn't have a neutral?

3) If the low voltage wire is connected to my furnace fan blower, how is power getting to the bathroom exhaust fan?

Any insight here would be greatly appreciated.

Switch wiring

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  • Can you rerun either of the cables that feed this box? A relay will allow you to hook a timer up to this mess, but needs a neutral or return to work, irrespective of whether it's line-voltage or low-voltage...
    – ThreePhaseEel
    Commented Sep 29, 2016 at 21:01

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This looks like a standard double pole switch.

double pole switch

Essentially, it is two independent switches both controlled by a single toggle. The two sides of switches are not connected to each other.

In your box, it looks like you have two separate switch loops, one going to each side of the switch. The cable on the left is supplying a hot lead an a return switched hot (probably to the 120V bathroom fan). The white wire is not a neutral, it is being used as a hot. Code requires that the white be marked with black tape or a marker to show it is hot.

The right cable (probably 24V furnace) uses red and black for the hot and the switched hot. This is standard color code. It seems that the right wire (lighter gauge) is the low voltage circuit.

The specific questions:

  1. There is no transformer in the switch. The low voltage is provided by a transformer elsewhere, probably near the furnace.
  2. There does not appear to be any neutral wire in this box. While that does not conform to current code, up until recently it was common to run a switch loop with only hot wires, using the white wire (properly marked) as a hot. Current code requires running a true neutral to each switch box (with a few exceptions), even if it is not currently needed.
  3. It is likely that power is provided directly to the box that the fan is connected to, but the hot line is not connected directly to the fan, it is connected to one of the wires running to this switch and then back to the fan on the other wire from this switch box.

While this setup works, I am not sure if current code allows a high and low voltage switch combination, or even high and low in the same box without a barrier between the sides. (NEC 725.136(B))

 Images and links are illustrative only, not an endorsement of goods or sources.
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  • That makes a tremendous amount of sense (the two switch loops through a single switch). As for the code implications, I live in Canada, so we're not covered under the NEC, but I expect this may not adhere to our code either. I'll confirm with my municipality. One follow-up question: If I wanted to rig this to a timer, I'd need to install one timer for each circuit, correct? As far as I can tell, the only way to install a single timer would be to marret both hots together, which would likely cause all sorts of problems, given that my furnace circuit is only 24V.
    – Aaron Hoyland
    Commented Sep 29, 2016 at 19:36
  • NO, NO, NO on combining hots from two different voltage sources! Standard practice would require two separate timers in two separate boxes (or they could they could be side by side in a double box with a separator).
    – bib
    Commented Sep 29, 2016 at 20:18
  • Exactly what I was thinking. I'll confirm code requirements in my locale and go from there. Thanks very much for the insight.
    – Aaron Hoyland
    Commented Sep 29, 2016 at 20:58

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